1. Field of the Invention
The present invention relates to an oxynitride piezoelectric material and a method of producing the oxynitride piezoelectric material, and more particularly to a piezoelectric material free of lead. Further, the present invention relates to a piezoelectric material including a perovskite-type oxynitride.
2. Description of the Related Art
Piezoelectric materials are widely used for applications, such as an actuator, an ultrasonic transducer, a micro-current source, and a generator of high piezoelectricity. Many of the piezoelectric materials used for those applications are each a material that is referred to as a so-called PZT, which is an oxide containing lead (Pb), zirconium (Zr), and titanium (Ti). Therefore, in view of environmental problems, a lead-free oxide piezoelectric material is being developed.
For example, as a lead-free oxide, barium titanate (BaTiO3) is known. However, barium titanate has a low Curie point of 120° C., and the piezoelectric properties thereof are eliminated at a temperature higher than 120° C. Accordingly, barium titanate is impractical when considering the applications in which the material containing barium titanate is exposed to high temperature, e.g., in a bonding step using solder or the like, and for in-vehicle use. In this way, because barium titanate has a low Curie point, the use of a lead-free oxide as an alternative to a lead oxide has not yet been achieved.
On the other hand, it is known that perovskite-type oxides include oxynitrides in addition to oxides. As illustrated in FIG. 1, YSiO2N and YGeO2N are being studied as oxynitrides in which N atoms are present in a state oriented in a c-axis direction (R. Caracas et al., Applied Physics Letters, Vol. 91, 092902 (2007)). According to the study, it has been revealed that high piezoelectric properties can be expected due to the ferroelectricity exhibited by N atoms oriented in the c-axis direction.
Further, in general, a piezoelectric material having a c-axis length longer than an a-axis length tends to have a higher Curie temperature. A ratio c/a (ratio of a c-axis length to an a-axis length in a unit cell) of YSiO2N is 1.37 and a ratio c/a of YGeO2N is 1.41, which are large (for example, a ratio c/a of BaTiO3 is about 1.01). Hence, the Curie points of YSiO2N and YGeO2N are expected to be high. In this manner, the oxynitrides have a potential to be used as a promising piezoelectric material.
However, a material in which the ratio c/a is too large has an unstable structure. Hence, this material is known to be difficult to produce. For example, BiCoO3 having a ratio c/a of 1.27 can only be produced under ultrahigh pressure and high temperature conditions. Furthermore, YSiO2N and YGeO2N each have a ratio c/a of far greater than 1.27. Therefore, they are extremely difficult to produce. There has been no example of the materials that were actually produced.
The present invention has been achieved in view of the above-mentioned problems, and provides a novel oxynitride piezoelectric material, which exhibits ferroelectricity and has good piezoelectric properties. It also provides a method of producing the oxynitride piezoelectric material.